The present invention relates to a potential analyzer for analyzing a potential at a surface of an object to be analyzed (for example, a potential in an LSI device) using an electron beam as a probe.
It is known that a potential at a portion where the electron beam is radiated can be measured with a scanning electron microscope by equipping it with an apparatus for analyzing energy of secondary electrons (refer to Japanese Patent Publication No. 51024/72).
FIG. 1A shows this principle. A retarding grid G is arranged between a specimen and a secondary electron detector 4 disposed to face the specimen 1. The retarding grid G forms a potential barrier to discriminate energy of secondary electrons 3 emitted from the specimen 1 due to its irradiation with an electron beam 2. FIG. 1B is a diagram showing the operation of this potential barrier. In the case where the retarding grid G is not used, every secondary electron 3 is detected by the secondary electron detector 4. An energy distribution of the secondary electrons emitted from the specimen 1 at zero potential is as shown by A in FIG. 1B. When the potential at the specimen 1 is at -5 V, the resulting secondary electron energy distribution is as indicated by B. When the retarding grid G is provided and a voltage of -5 V is applied thereto, secondary electrons to be detected are limited to those having 5 eV or more, so that a change occurs in the detection quantity of secondary electrons to be detected in dependence upon the potential at the specimen 1. In this way, since the secondary electron detection quantity relates to the potential at the specimen, the potential at the specimen 1 can be known from the detection quantity of secondary electrons.
However, in this method of analyzing the potential on the basis of only the arrangement of the retarding grid G, it is difficult to quantitatively analyze the potential since there is not a linear relation between the potential at the specimen 1 and the detection quantity of secondary electrons. Therefore, for linearization of the above-mentioned relation, a feedback loop operation is known in which the potential at the retarding grid is adjusted by a specific circuit so as to always maintain the detection quantity of secondary electrons constant (H. P. Feurbaum et al, IEEE Journal of solid state circuits, Vol. SC-13, No. 3, 1978).
FIG. 2 is a block diagram to describe this feedback loop operation. An output of the secondary electron detector 4 is compared with a reference voltage 6 and the difference is amplified by an amplifier 5 and its output is given to the retarding grid G. Since the potential at the retarding grid G decreases in association with an increase in the detection quantity of secondary electrons, even if the potential at the specimen 1 changes arbitrarily, the secondary electron detection quantity will be kept constant. Since an amount of change in the potential at the specimen is made to have a one-to-one correspondence to an amount of change of the retarding grid G, a change in the potential at the specimen 1 can be quantitatively known by measuring the potential at the retarding grid G.
However, to accurately perform this measurement, a variable range of the potential at the retarding grid G has to be set within a predetermined range. The gain of the secondary electron detector 4 is usually adjusted to make the detector 4 operative within this range. However, if the intensity of the primary electron beam changes or if the material is different from a specimen to another, this operating point will be changed, which will cause a large error in measurement. For example, in the case of measuring voltage waveforms on circuit lines in an LSI, there are a number of points to be measured therein and the resulting voltages are compared with each other. However, the materials of the circuit lines in the LSI are not always identical (for instance, Al, poly-Si, gold, etc.). Due to this, an emission efficiency of secondary electrons differs, so that the gain of the secondary electron detector 4 has to be adjusted to match with a specified operating point (potential at the retarding grid G). This adjustment is needed almost whenever the measurement point is changed. Namely, in the foregoing method, the adjustment has to be performed while always paying attention to the operating point, which makes this method complicated and brings about measurement errors therein.